Children with mental retardation are in special need of ways of facilitating their transition to linguistic communication. The transactional model of development and recently discovered empirical evidence results in the expectation that both maternal responsivity and clear prespeech communication must be considered when accounting for individual differences in the ages at which this transaction occurs. Only a study with a longitudinal experimental design can determine whether maternal responsivity and frequent, clear prespeech communication facilitate later language development. To manipulate maternal responsivity and frequent, clear prespeech communication, a combination of Responsivity Training (RT) for mothers and Prespeech Milieu Teaching (PMT) for children, called combined Training (CT) will be used. Eight hypotheses will be tested. First, it is predicted that CT will increase clear prespeech communication in children with mental retardation and responsiveness to children's communication in their mothers. Second, it is predicted that increasing the frequency of clear, prespeech communication and maternal responsivity will result in increased receptive vocabulary, receptive semantic relation ability, productivity vocabulary, and the extent to which communication is verbal. Third, it is predicted that CT's effect on linguistic outcomes will be mediated by posttreatment clear prespeech communication and responsivity. Fourth, to explore other possible causal models for the CT effects on linguistic outcomes, whether facilitating responsivity and clear, prespeech communication indirectly improves the home environment in more general ways and mediated CR effects on linguistic outcomes will be tested. Fifth, in the unlikely event there are no main effects on productive linguistic outcomes, the prediction that manipulating clear, prespeech communication and maternal responsivity affects these outcomes if the child shows sufficiently complex vocal abilities will be tested. Sixth, in the unlikely event there are no main effects on prespeech communication and responsivity, whether pretreatment SES, perception of adequacy of resources, and responsivity moderates the treatment effect will be tested. Seventh, in the unlikely event prespeech communication and responsivity do not facilitate productive vocabulary, whether the number of complex vocal acts interacts with maternal responsivity to predict later productive vocabulary will be tested. Eighth, whether CT is superior to PMT alone for mothers with relatively low pretreatment responsivity will be tested if a sufficient number of children can be matched from the existing PMT alone group and the proposed CT group at the pretreatment period.